1. Field of the Invention
The present invention relates to a surface mounting type light emitting display device. The present invention also relates a process for making such a display device, and a surface mounting structure incorporating the same.
2. Description of the Related Art
Light emitting display devices are widely used for representing figures, characters, signs and the like in electric equipments such as portable telecommunication devices and audio-visual apparatuses. A typical example of light emitting display device is a lead mounting type light emitting display device, as shown in FIGS. 11a and 11b of the accompanying drawings.
Specifically, the lead mounting type light emitting display device shown in FIGS. 11a and 11b comprises a case member 22 having a plurality of segmented display windows 21 in a predetermined arrangement (figure eight for example), and a circuit board 24 housed in the case member 22. The case member 22 may be made of e.g. polycarbonate containing a coloring pigment. The circuit board 24, which is made of glass-fiber-reinforced epoxy resin, carries a plurality of light emitting diodes (LED) 25 in corresponding relation to the segmented display windows 21, and has a plurality of downwardly projecting leads 26. Each of the LEDs 25 has a lower electrode layer (not shown) bonded to a predetermined portion of the circuit pattern of the circuit board 24 in electrical conduction therewith. The LED 25 also has an upper electrode layer (not shown) electrically connected to another predetermined portion of the circuit pattern through a metal wire 27. The interior space of the case member 22 and the segmented display windows 21 are filled with a body 23 of transparent resin (mainly containing epoxy resin for example). The transparent resin body 23 may be defoamed before hardening thereof to prevent light deflection at the interfaces between the foams and the resin.
In use, the light emitting display device described above is mounted to a suitable portion of a mother circuit board (not shown) together with other electronic components (not shown). Most of the other electronic components is of the leadless surface mounting type due to the need for size reduction. Thus, the majority of the other electronic components may be mounted on the mother circuit board by reflow soldering, whereas the lead type light emitting display device need be mounted to the mother circuit board by manual soldering or dip-soldering in a separate process step. Further, the mother circuit board need be made to have through-holes for allowing insertion of the leads of the display device, and a subsequent process step is additionally required for cutting or bending the inserted leads.
It is conceivable, therefore, to modify the lead type light emitting display device into a leadless surface mounting type light emitting display device by replacing the leads with layer-like terminal electrodes. Such a modification leads to the following new problems when reflow soldering is performed for the display device.
First, since the case member 22, the circuit board 24 and the transparent resin body 23 are all made of different materials, they have different coefficients of linear expansion, which leads to thermal warping of the display device at the time of reflow soldering. More specifically, when the display device is mounted by reflow soldering, the display device is heated to a high temperature of about 250.degree. C., as opposed to manual soldering or dip-soldering wherein the display device is heated only to a temperature of less than 100.degree. C. Thus, there will be a significant difference in thermal expansion between the case member 22, the circuit board 24 and the transparent resin body 23 if these parts are firmly fixed together. As a result, the display device will warp due to the so-called "bimetal phenomenon" to be partially lifted off the mother circuit board, thereby causing improper electrical connection relative to the mother circuit board.
Secondly, since each of the LEDs 25 together with its associated wire 27 is completely enclosed within the transparent resin body 23 in direct contact therewith, the LED 25 and the wire 27 may be mechanically damaged (e.g. cutting or separation of the wire) due to thermal expansion of the transparent resin body 23.
The above-described problems become particularly remarkable if the size of the display device is enlarged to accommodate a large number of segmented display windows.